Location and Fair Values of Derivative Financial Instruments (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Derivative
Asset Derivatives Fair Value	$ 20.6	$ 32.7
Designated as Hedging Instrument | Other Long-term Investments
Derivative
Asset Derivatives Fair Value	20.6	32.7
Designated as Hedging Instrument | Other Liabilities
Derivative
Liability Derivatives Fair Value	24.2	20.3
Designated as Hedging Instrument | Interest Rate Swaps | Other Long-term Investments
Derivative
Asset Derivatives Fair Value	0.0	0.0
Designated as Hedging Instrument | Interest Rate Swaps | Other Liabilities
Derivative
Liability Derivatives Fair Value	3.8	6.9
Designated as Hedging Instrument | Foreign Exchange Contracts | Other Long-term Investments
Derivative
Asset Derivatives Fair Value	20.6	32.7
Designated as Hedging Instrument | Foreign Exchange Contracts | Other Liabilities
Derivative
Liability Derivatives Fair Value	20.4	13.4
Not Designated as Hedging Instrument | Other Liabilities
Derivative
Liability Derivatives Fair Value	53.2	79.2
Not Designated as Hedging Instrument | Interest Rate Swaps | Other Liabilities
Derivative
Liability Derivatives Fair Value		0.7
Not Designated as Hedging Instrument | Foreign Exchange Contracts | Other Liabilities
Derivative
Liability Derivatives Fair Value	27.7	31.4
Not Designated as Hedging Instrument | Credit Risk Contract | Other Liabilities
Derivative
Liability Derivatives Fair Value	0.3	0.4
Not Designated as Hedging Instrument | Embedded Derivative in Modified Coinsurance Arrangement | Other Liabilities
Derivative
Liability Derivatives Fair Value	$ 25.2	$ 46.7